1. Field of the Invention
The present invention relates to cross-linked, imide-based, photomechanical polymers derived from a multi(azobenzene-amine) cross-linker in conjunction with an aromatic amine and an aromatic dianhydride.
2. Description of the Related Art
Photomechanical polymers are a special class of smart (stimuli-responsive) polymers that are responsive to light and are capable of generating photo-directed motions and dimensional or shape alteration at the macro-scale level. These motions and/or alterations are driven by the collective molecular-volume change brought about by the chromophoric units upon appropriate irradiation. The chromophoric units in photoresponsive polymers are photochromic and have the unique ability to reversibly interconvert between two structural isomers (each with distinctly different optical and physical properties) under appropriate excitation conditions. Examples of chromophoric units can be found in photo-isomerizable molecules such as azobenzenes, spiropyrans, spirooxazines, diarylethylenes, and fulgides. However, with respect to photomechanical polymers, azobenzene is the most commonly used molecularly-actuating moiety because of its excellent thermal stability and resolved isomeric forms, as well as its ability to form surface gradient reliefs when subjected to conventional or polarization holography.
The resulting photomechanical output of a polymeric material depends not only on its optical properties (absorption wavelength, wavelength of exposure, polarization of exposure), but also on its molecular architecture and morphology (amorphous, crystalline, liquid crystalline). In addition, the photomechanical output is dependent on the polymeric material's thermomechanical properties, as well as the geometrical properties of the device, e.g. thickness of a cantilever.
Polyimides (PIs) represent an important class of heat-resistant polymers that have found utility in a wide spectrum of applications, ranging from structural components to electronic and photonic devices. The widespread application of polyimides is mainly the result of their excellent combination of physical properties, thermal stability, and processability. For example, polyimides containing azobenzene in the backbone or side-chain have been investigated for photo-induced alignment in liquid crystal display (LCD) as well as nonlinear optical applications. More recently, an azobenzene-containing poly(amic acid) (a PI precursor) was crosslinked by a triamine in N,N-dimethylformamide (DMF) and the resulting sol-gels showed a two-fold increase in the storage modulus after irradiation with 405 nm light. Another closely related aromatic poly(amic acid) was reported to be photomechanically active.